1. Field of the Invention
The present invention relates to a process for producing a biodegradable and/or absorbable silica sol material which is advantageous especially with regard to the reproducibility of the material, the speed of the synthesis and the possibility of being able to produce the silica sol material on an industrial scale. A further subject matter of the invention relates to a biodegradable and/or absorbable silica sol material which can be produced by the synthesis process according to the invention.
2. Description of Related Art
Biodegradable and/or absorbable silica sol materials and the production thereof are described in the prior art.
DE 196 09 551 C1 describes biodegradable/bioabsorbable fibre structures which can be obtained in a sol gel process by drawing threads from a spinning mass and optionally drying them. The production of the sol material involves a hydrolysis-condensation step in which the starting materials tetraethoxysilane (TEOS), ethanol, water and nitric acid are mixed in the molar ratio 1:1.26:x:0.01 (where x=1.6, 1.7, 1.8, 1.9 or 2.0). DE 196 09 551 C1 describes how the required water/acid mixture is added directly to the silicon compound to be hydrolysed (TEOS) at room temperature or with slight cooling and the resulting mixture is then stirred for one to several hours. When the hydrolysis is complete, solvent is removed from the resulting mixture until the mixture has a viscosity between 0.05 and 50 Pa·s at room temperature and a shear gradient of 20 s−1. Preferably, filtration is then carried out. The filtrate is ripened in a closed vessel at a temperature of 3° C. and a ripening time of from 6 hours to 6 months to give the silica sol material. The silica sol material can then be spun to give a fibre.
The addition of an aqueous acid to the Si compound located in a solvent can result in the formation of mist in the reaction vessel. The particles which are found in such a mist can subsequently be removed only by means of a filtration step. If the particles are not removed via a filtration step, they may, for example during the further processing of the silica sol material to give a silica gel fibre by means of a spinning apparatus, block the spinning jets of the spinning apparatus. The spinning process then has to be interrupted and the jets have to be cleaned. On the other hand, an additional filtration step is to be avoided as far as possible since synthesis material is lost with the filtration which can no longer be reacted to give the silica sol material.
WO2008/086970A1 relates to a similar silica sol material for which, however, the hydrolysis-condensation step is carried out over a period of at least 16 hours. The hydrolysis-condensation reaction is preferably carried out discontinuously in a stirred container. The Si compound and the solvent are preferably introduced as initial charge. An acid, preferably in the form of HNO3, is then added swiftly. It is described how the hydrolysis-condensation reaction proceeds rapidly on account of the acid strength and the contents of the container heat up by about 40° C. Subsequent removal of the solvent is carried out until the viscosity of the mixture is from 0.5 to 2 Pa·s at a shear rate of 10 s−1 at 4° C. A filtration is allegedly not required. The ripening follows at a temperature of preferably 2° C. to 4° C. in order to bring about a further condensation under kinetically controlled conditions, the aim being to suppress the formation of a three-dimensional polymeric gel network. The product of the ripening preferably has a viscosity of from 35 to 45 Pa·s (shear rate 10 s−1 at 4° C.) with a loss factor (at 4° C., 10 s−1, 1% deformation) of from 2.5 to 3.5.
WO2008/148384A1 relates to a similar silica sol material. When producing this material, a gas-diffusion-tight container, preferably a rotary evaporator, is used during the evaporation. The hydrolysis-condensation step involves the direct addition of a mixture of H2O and HNO3 to a mixture of TEOS and ethanol. The kinetically controlled ripening is carried out in particular by means of vibration-free storage of the reaction mixture in a closed gas-diffusion-tight vessel.
WO2009/077104A relates to a similar silica sol material for which, in contrast to WO2008/148384A1, the evaporation takes place in a closed apparatus optionally by means of a continuous introduction of a chemically inert stream of entrainer gas.
All of the specified processes from the prior art involve at least three steps: hydrolysis-condensation, solvent removal and ripening. The steps are obviously carried out in different vessels. The times for the individual steps sometimes vary considerably. The individual steps are complete when a preferred viscosity has been established; i.e. it is obviously necessary to monitor the viscosity of the reaction mixtures in order to be able to end a step upon the presence of the desired (intermediate) product.
In the specified processes from the prior art, the hydrolysis-condensation reaction takes place with the rapid bringing together of the silicon compound to be hydrolysed and the acid. As described in each case, the exothermy of the reaction results in a considerable increase in temperature. Such a temperature increase is to be avoided especially in the case of an up-scaling to an industrial production on a large scale inter alia for safety reasons. Moreover, as a result of temperature inhomogeneities within the reaction mixture, product inhomogeneities can arise particularly on an industrial scale.
According to the prior art, the ripening preferably takes place vibration-free at a temperature of −20° C. to 10° C. and it can last from 6 hours to 6 months. Such a process is not scaleable and is therefore unsuitable for large-scale production.
On account of the inadequate controllability of the processes described in the prior art, the end of the silica sol material production process at the start of the production can only be predicted within a range of ±2 days. Particularly with regard to the biodegradable or bioresorbable properties of the materials, which possibly permit only short storage times, the uncertainties described above are not conducive to useful economic planning and production.